Organic electroluminescence layer with oligomer hole injection layer

ABSTRACT

The present invention is to provide an organic EL element which can secure the durability by which to endure the pulse driving and which has an excellent heat resistance. In an organic electroluminescence element in which a luminous layer and a hole injection layer are formed between an anode and a cathode, the hole injection layer is formed using an oligomer having a phenylenediamine structure and having a glass transition temperature of 110° C. or more, and an intermediate layer is formed between the hole injection layer and the anode to inhibit a chemical reaction in an interface between the anode and the hole injection layer. Consequently, it is possible to remarkably improve the heat resistance of the element, to secure the excellent durability capable of enduring severe driving conditions, and to realize the prolongation of life of the element.

This application is a 371 of PCT/JP99/02377, filed May 7, 1999.

TECHNICAL FIELD

The present invention relates to an organic electroluminescence elementin which at least a hole injection layer and a luminous layer areinterposed between a pair of electrodes.

BACKGROUND OF THE INVENTION

Since an electroluminescence element (hereinafter referred to as an “ELelement”) using electroluminescence causes self-luminescence, thevisibility is high and the element is a completely solid state elementso that it has an excellent impact strength. Accordingly, its use as aluminescence element in various display devices has attracted muchinterest.

This EL element includes an inorganic EL element obtained by using aninorganic compound as a luminous material and an organic EL elementobtained by using an organic compound as a luminous material. Especiallyin the organic EL element of these, the driving voltage is quite low ascompared with the inorganic EL element, and the element can furthereasily be downsized. Accordingly, studies and developments to put thesame to practical use have been increasingly conducted.

In order to put the organic EL element to practical use, the increase inefficiency of the element performance and the improvement in the drivinglife are indispensable. For solving these problems, the improvements inthe luminous material and the construction of the element have beencarried out.

As the organic EL element, known is an element based on a laminated-typeelement construction of anode/organic luminous layer/cathode andprovided on this with a hole injection transfer layer or an electroninjection transfer layer as required, for example, an element having theconstruction of anode/hole injection transfer layer/organic luminouslayer/cathode or the construction of anode/hole injection transferlayer/organic luminous layer/electron injection transfer layer/cathode,or the like is known.

The hole injection transfer layer here has a function of injecting holesfrom an anode at good efficiency and transferring the holes to theluminous layer, and it is, in many cases, constructed of a holeinjection layer and a hole transfer layer.

Further, the electron injection transfer layer has a function ofinjecting electrons from a cathode at good efficiency and transferringthe electrons to the luminous layer.

The luminous layer has a function of conducting luminescence byrecombination of holes and electrons injected.

As the anode of the organic EL element, a transparent electrode formedof an ITO (Indium Tin Oxide) film is generally used. In this case, inorder to inject holes from ITO at good efficiency by reducing an energybarrier in the hole injection, an amine-type material of which theionization potential is close to that of ITO to give a great degree ofhole transfer is often used in the hole injection layer.

By the way, since the organic EL element has quite a low thickness ofapproximately 100 nm, the surface form of ITO greatly influences theperformance of the element. Specifically, when a protrusion or the likeis present on the surface of ITO, the crystallization of the organicthin film proceeds with this protrusion as a base point, and it causesincrease in a leak current or formation of a non-luminous point called adark spot. For this reason, a high amorphousness and good filmproperties are required for the hole injection layer formed on ITO.

Further, when the organic EL element is driven through constant currentdriving, the driving voltage is increased with time to decrease aluminance. Such a deterioration phenomenon is considered to occurbecause a chemical reaction such as an oxidation reaction or the like iscaused in an interface between ITO and the hole injection layer which isdirectly contacted with this ITO to proceed with the deteriorationthrough the driving.

In order to solve such a problem, an organic EL element in which a holeinjection transfer zone is constructed of a layer containing ahole-injecting porphyrin compound and a hole-transferring aromatictertiary amine is disclosed (U.S. Pat. No. 2,597,377).

Further, it has been known that a CuPc (copper phthalocyanine) thin filmis formed on the surface of ITO whereby a driving stability can beincreased to reduce the increase in the driving voltage (S. A. Van Slykeet al., Appl. Phys. Lett., 69, 2160 (1996)).

Japanese Patent Laid-Open No. 314594/1994 discloses a structure in whicha CuPc film is formed on ITO and a hole injection layer formed of aTPD-based oligomer, a triarylamine derivative is laminated on this CuPcfilm.

Although the driving stability is improved by these methods, there was aproblem that a driving life (durability) cannot be improvedsatisfactorily.

Further, when pulse driving such as simple matrix driving is employed asa driving system in applying an organic EL element to a dot matrixdisplay or the like, a current density has to be increased forconducting luminescence instantaneously at high luminance. Accordingly,there arises a need to pass a great current by periodically applying ahigh voltage. As a result, an element comes to be driven under severerconditions than in case of ordinary DC (direct current) driving.Therefore, the pulse driving involved a problem that a chemical reactionin an interface between ITO and a hole injection layer tends to proceedto give a short life.

Further, in order to put a luminescence element to practical use, astability at a high temperature is required. However, when an ordinaryorganic EL element is stored at a high temperature, there are problemsthat the efficiency tends to decrease, further luminescence becomesuniform and the like. Thus, it was difficult to put the same topractical use.

It is an object of the present invention to provide an organic ELelement in which a durability by which to endure the pulse driving canbe secured and the heat resistance is excellent.

DISCLOSURE OF THE INVENTION

The present inventors have assiduously conducted investigations, andhave consequently obtained findings that an intermediate layer isinterposed between a hole injection layer and an anode and a material tomeet predetermined conditions is used in the hole injection layer,making it possible to realize the prolongation of life and theimprovement in the heat resistance. The present invention has beencompleted on the basis of these findings.

That is, the gist of the present invention is as follows.

1. An organic electroluminescence element comprising an anode and acathode which are opposite to each other, and a hole injection layer anda luminous layer which are interposed between these anode and cathode,characterized in that the hole injection layer contains an oligomerhaving a phenylenediamine structure and having a glass transitiontemperature of 110° C. or more, and an intermediate layer for inhibitinga reaction in an interface between the hole injection layer and theanode is formed between the hole injection layer and the anode.

2. The organic electroluminescence element as recited in the above 1,wherein an ionization potential of the intermediate layer is larger thana work function of the anode and smaller than an ionization potential ofthe oligomer of the hole injection layer.

3. The organic electroluminescence element as recited in the above 1,wherein the intermediate layer is formed of an inorganic semiconductor.

4. The organic electroluminescence element as recited in the above 2,wherein the intermediate layer is formed of an inorganic semiconductor.

5. The organic electroluminescence element as recited in the above 1 or2, wherein the intermediate layer is formed of an inorganic insulator.

6. The organic electroluminescence element as recited in the above 1 or2, wherein the intermediate layer is formed of a phthalocyanine-basedcompound.

7. The organic electroluminescence element as recited in the above 1 or2, wherein the intermediate layer is formed of a carbon film.

The present invention is an organic electroluminescence elementcomprising an anode and a cathode which are opposite to each other, anda hole injection layer and a luminous layer which are interposed betweenthese anode and cathode, characterized in that the hole injection layercontains an oligomer having a phenylenediamine structure and having aglass transition temperature of 110° C. or more, and an intermediatelayer for inhibiting a reaction in an interface between the holeinjection layer and the anode is formed between the hole injection layerand the anode.

The hole injection layer here is a layer which is formed between theanode and the luminous layer for improving the injection property of thehole.

Further, the phenylenediamine structure here is a structure that twoamines are arranged through a phenyl group.

Examples of the material having this structure include, for example, acompound represented by general formula (I)

(wherein n is an integer of 1 to 3, Ar¹ to Ar⁷ each represent acarbocyclic group having 6 to 30 carbon atoms, and either Ar² or Ar⁵ isa phenylene group), a compound represented by general formula (II)

(wherein m is an integer of 1to 3, Ar¹ to Ar⁹each represent acarbocyclic group having 6 to 30 carbon atoms, and at least one of Ar²,Ar⁴ and Ar⁵ is a phenylene group), and the like.

In the present invention, among the oligomers having such aphenylenediamine structure, one having a glass transition temperature of110° C. or more is used as the material for the hole injection layer, sothat the heat resistance of the element can markedly be improved and theexcellent luminous efficiency is obtained.

And, the intermediate layer is formed between the hole injection layercontaining such an oligomer and the anode, whereby the hole injectionlayer and the anode can be spaced apart, making it possible to eliminatethe chemical reaction in the interface between the hole injection layerand the anode. Consequently, the excellent durability by which to endurethe severe driving conditions in the pulse driving or the like can besecured, and the prolongation of life of the element can be achieved.

The hole injection layer may be interposed between the anode and theluminous layer along with the hole transfer layer, and may have afunction as a hole injection layer that improves both the injectionproperty and the transferability of the hole.

The above-described oligomer may be a linear oligomer or a branchedoligomer.

In this case, it is desirable that the ionization potential of theintermediate layer is larger than a work function of the anode andsmaller than the ionization potential of the oligomer of the holeinjection layer.

When the ionization potential of the intermediate layer is thus defined,the hole injection barrier can surely be decreased, making it possibleto decrease the driving voltage and to improve the durability.

The material constituting the intermediate layer may be either anorganic material or an inorganic material. For example, the intermediatelayer can be formed of an inorganic semiconductor or an inorganicinsulator. Examples of the inorganic semiconductor include, for example,GaAlN, GaInN, GaN, Si_(x−1)—C_(x) (0<×<1), Si, CuI, ZnTe, ZnS, CdS,CdTe, CdSe_(x)S_(1−x)(0<×<1), Te and Se. Examples of the inorganicinsulator include SiO_(x) (0<×<2), LiF, Li₂O, Al₂O₃, TiO₂, BaF₂, CaF₂,MgF₂ and the like.

In case the organic material is used as the material of the intermediatelayer, the intermediate layer can be formed of a phthalocyanine-typecompound, a quinacridone-type compound or the like.

Further, the intermediate layer can also be formed of a carbon film, andit can specifically be formed of p-type diamond, a diamond-like carbonfilm (SP³ ingredient-containing carbon film) or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing one embodiment of the organicelectroluminescence element in the present invention. In FIG. 1, numeral10 is a transparent substrate, numeral 11 an anode, numeral 12 acathode, numeral 13 a luminous layer, numeral 14 a hole injection layer,and numeral 15 an intermediate layer.

BEST MODE FOR CARRYING OUT THE INVENTION

A mode for carrying out the present invention is described below byreferring to the drawings.

FIG. 1 shows an organic EL element 1 of the present mode.

The organic EL element 1 of the present mode has an element structure inwhich the transparent substrate 10 is used as a support and the luminouslayer 13 and the hole injection layer 14 formed of the organic materialare interposed between the pair of the electrodes 11, 12 opposite toeach other, and the transparent substrate 10 is used as a light draw-outsurface.

Of the pair of the electrodes 11, 12, the anode 11 is made of atransparent electrode such as an ITO thin film or the like, and formedon the transparent substrate 10 made of a glass or the like.

The hole injection layer 14, the luminous layer 13 and the cathode 12are laminated on this anode 11 in this order, and the intermediate layer15 is interposed between the hole injection layer 14 and the anode 11.

The hole injection layer 14 is constructed by containing the straightchain oligomer or the branched chain oligomer having thephenylenediamine structure and having a glass transition temperature of110° C. or more.

The intermediate layer 15 is a layer for inhibiting the reaction in theinterface between the hole injection layer 14 and the anode 11, and itis formed of the material of which the ionization potential is largerthan the work function of the anode 11 and smaller than the ionizationpotential of the oligomer of the hole injection layer 14.

Such an intermediate layer 15 is formed of any of the inorganicsemiconductor, the phthalocyanine-based compound and the carbon film.

The layer construction of the organic EL element in the presentinvention may be one containing the luminous layer and the holeinjection layer, and it is not particularly limited. For example, it canbe applied to organic EL elements having various layer constructions,for example, anode/intermediate layer/hole injection layer/organicluminous layer/electron injection transfer layer/cathode,anode/intermediate layer/hole injection layer/hole transferlayer/organic luminous layer/electron injection transfer layer/cathode,and the like.

Next, the effects of the present invention are described on the basis ofspecific Examples.

EXAMPLES 1

In this Example 1, an organic EL element was obtained by employing thefollowing specific element construction in the above-described mode.

Incidentally, the following layers (2) to (5) were formed by vacuumdeposition.

(1) Construction of an element

1) anode: ITO

2) intermediate layer: CuPc(copper phthalocyanine)

3) hole injection layer: laminated film of TPD 87 and TPD 78

4) luminous layer: Alq (tris(8-quinolinol) aluminum)

5) cathode: Composite film of Al and Li

The structural formulas of TPD 87 and TPD 78 are as follows.

EXAMPLE 2

An organic EL element of this Example 2 was produced as in Example 1using the same construction of the element as in Example 1 except thatthe intermediate layer was formed of a carbon film.

EXAMPLE 3

An organic EL element of this Example 3 was produced as in Example 1using the same construction of the element as in Example 1 except thatthe intermediate layer was formed of an SiO₂ film. However, the SiO₂film was formed by a sputtering method.

Comparative Example 1

An organic EL element of this Comparative Example 1 was obtained as inExample 1 employing the same construction of the element as in Example 1except that the intermediate layer was omitted.

Comparative Example 2

An organic EL element of this Comparative Example 2 was obtained as inExample 1 using the same construction of the element as in Example 1except that the hole injection layer was a laminated film of TPD 74 andTPD 78.

The structural formula of TPD 74 is as shown below.

Comparative Example 3

An organic EL element of this Comparative Example 3 was obtained as inExample 1 employing the same construction of the element as in Example 1except that the intermediate layer was omitted and the hole injectionlayer was a laminated film of TPD 1 and TPD 2.

The structural formulas of TPD 1 and TPD 2 are as shown below.

Comparative Example 4

An organic EL element of this Comparative Example 3 was obtained as inExample 1 using the same construction of the element as in Example 1except that the hole injection layer was a laminated film of TPD 1 andTPD 2.

Incidentally, a work function of ITO is 5.1 eV, and ionizationpotentials of the materials for the intermediate layer and the holeinjection layer used in Examples 1 and 2 and Comparative Examples 1 to 4are as shown in Table 1.

TABLE 1 Ionization Material potential (eV) Hole injection TPD 87 5.2layer TPD 78 5.5 TPD 74 5.2 TPD 1 5.4 TPD 2 5.5 Intermediate layer CuPc5.2 C 5.2

Evaluation of an Organic EL Element

With respect to the organic EL elements of Examples 1 to 3 andComparative Examples 1 to 4, the durability and the heat resistance weremeasured respectively.

The measurement of the durability was conducted by driving each of theorganic EL elements with the initial luminance of 300 cd/m² andmeasuring the half-life. At this time, the driving conditions are a dutyof {fraction (1/100)} and a driving frequency of 60 Hz. The results areshown in Table 2.

Incidentally, in Table 2, the life of each element was shown in terms ofa ratio given when the half-life in Comparative Example 1 was defined as1.

The measurement of the heat resistance was conducted by storing each ofthe organic EL elements at 85° C. for 500 hours and examining the changein the luminous efficiency before and after the storage. The results areshown in Table 2. In Table 2, o indicated that the luminous efficiencywas not decreased, and x indicated that the luminous efficiency wasdecreased.

TABLE 2 Interme- Heat diate Hole injection resis- layer layer Tg (° C.)*Life tance Example 1 CuPc TPD 87/TPD 78 112/126 5 ∘ Example 2 C TPD87/TPD 78 112/126 4.5 ∘ Example 3 SiO₂ TPD 87/TPD 78 112/126 5 ∘Comapara- — TPD 87/TPD 78 112/126 1 ∘ tive Example 1 Compara- CuPc TPD74/TPD 78 80/126 3 x tive Example 2 Compara- — TPD 1/TPD 2 120/115 1 ∘tive Example 3 Compara- CuPc TPD 1/TPD 2 120/115 2.1 ∘ tive Example 4*Tg(° C) indicates a glass transition temperature of each materialconstituting a hole injection layer.

From Table 2, it is found that since the intermediate layer formed ofCuPc is provided in Example 1, the pulse life is prolonged more than inComparative Example 1 in which the intermediate layer is absent.

From Example 2 or 3, it is found that when the intermediate layer is acarbon film or SiO₂, the effect of improving the pulse life is obtained.

On the other hand, in Comparative Example 2, it is found that since thehole injection layer is formed of TPD 74 having the low glass transitiontemperature (Tg), the heat resistance is poor.

Comparative Example 3 is a case in which the hole injection layer isformed of TPD 1 and TPD 2 both having no phenylenediamine structure. Ithas the same life as Comparative Example 1.

In Comparative Example 4, the intermediate layer is formed on theelement of Comparative Example 3. However, it is found that since thematerial of the hole injection layer has no phenylenediamine structure,the pulse life is increased to a small extent in spite of the formationof the intermediate layer.

Industrial Applicability

As stated above, according to the present invention, the heat resistanceand the luminous efficiency can be improved by forming the holeinjection layer using the oligomer having the phenylenediamine structureand having the glass transition temperature of 110° C. or more.

Further, the intermediate layer is formed between the hole injectionlayer containing such an oligomer and the anode, whereby the excellentdurability by which to endure severe driving conditions of the pulsedriving or the like can be secured and the prolongation of life of theelement can be achieved.

What is claimed is:
 1. An organic electroluminescence element comprisingan anode and a cathode which are opposite to each other, and a holeinjection layer and a luminous layer which are interposed between theanode and the cathode, wherein the bole injection layer has a glasstransition temperature of 110° C. or more and comprises an oligomerhaving a phenylenediamine structure, wherein an intermediate layer forinhibiting a reaction in an interface between the hole injection layerand the anode is formed between the hole injection layer and the anode,and wherein an ionization potential of said intermediate layer is largerthan a work function of said anode and smaller than an ionizationpotential of the oligomer of said hole injection layer.
 2. The organicelectroluminescence element as claimed in claim 1, wherein saidintermediate layer is formed of an inorganic semiconductor.
 3. Theorganic electroluminescence element as claimed in claim 1, wherein saidintermediate layer is formed of an inorganic insulator.
 4. The organicelectroluminescence element as claimed in claim 1 wherein saidintermediate layer is formed of a phthalocyanine-based compound.
 5. Theorganic electroluminescence element as claimed in claim 1, wherein saidintermediate layer is formed of a carbon film.